In magnetic storage systems, a transducing head writes digital data onto a magnetic storage medium. The digital data serve to modulate the current in a read/write head coil so that a sequence of corresponding magnetic flux transitions are written onto the magnetic medium in concentric tracks. To read this recorded data, the read/write head passes over the magnetic medium and transduces the magnetic transitions into pulses in an analog signal. These pulses are then decoded by the read channel circuitry to reproduce the digital data.
Decoding the pulses into a digital sequence can be performed by a simple pulse detector in a conventional analog read channel or, as in more recent designs, by using a discrete time sequence detector in a sampled amplitude read channel. Discrete time sequence detectors are preferred over simple analog pulse detectors because discrete time detectors compensate for intersymbol interference (ISI), thereby decreasing the necessary bandwidth. Thus, more data can be stored on the storage medium. There are several types of well known discrete time sequence detection methods including discrete time pulse detection (DSP), maximum likelihood sequence detection (MLSD), decision-feedback equalization (DFE), enhanced decision-feedback equalization (EDFE), and fixed-delay tree-search with decision-feedback (FDTS/DF).
The application of sampled amplitude techniques to digital communication channels is well documented. See Y. Kabel and S. Pasupathy, "Partial Response Signaling", IEEE Trans. Commun. Tech., Vol. COM-23, pp.921-934, Sept. 1975; and Edward A. Lee and David G. Messerschmitt, "Digital Communication", Kluwer Academic Publishers, Boston, 1990; and G. D. Forney, Jr., "The Viterbi Algorithm", Proc. IEEE, Vol. 61, .pp. 268-278, March 1973. Applying sampled amplitude techniques to magnetic storage systems is also well documented. See Roy D. Cideciyan, Francois Dolivo, Walter Hirt, and Wolfgang Schott, "A PRML System for Digital Magnetic Recording", IEEE Journal on Selected Areas in Communications, Vol. 10 No. 1, January 1992, pp.38-56; and Wood et el, "Viterbi Detection of Class IV Partial Response on a Magnetic Recording Channel", IEEE Trans. Commun., Vol. Com-34, No. 5, pp. 454-461, May 1986; and Coker Et al, "Implementation of PRML in a Rigid Disk Drive", IEEE Trans. on Magnetics, Vol. 27, No. 6, Nov. 1991; and Carley et al, "Adaptive Continous-Time Equalization Followed By FDTS/DF Sequence Detection", Digest of The Magnetic Recording Conference, August 15-17, 1994, pp. C3; and Moon et al, "Constrained-Complexity Equalizer Design for Fixed Delay Tree Search with Decision Feedback", IEEE Trans. on Magnetics, Vol. 30, No. 5, Sept. 1994; and Abbott et al, "Timing Recovery For Adaptive Decision Feedback Equalization of The Magnetic Storage Channel", Globecom '90 IEEE Global Telecommunications Conference 1990, San Diego, Calif., Nov. 1990, pp.1794-1799; and Abbott et al, "Performance of Digital Magnetic Recording with Equalization and Offtrack Interference", IEEE Transactions on Magnetics, Vol. 27, No. 1, Jan. 1991; and Cioffi et al, "Adaptive Equalization in Magnetic-Disk Storage Channels", IEEE Communication Magazine, Feb. 1990; and Roger Wood, "Enhanced Decision Feedback Equalization", Intermag '90.
In disk drives utilizing either analog or sampled amplitude read channels, the read/write head is normally mounted on an actuator arm which is positioned by means of a voice coil motor ("VCM"). The VCM moves the head and actuator arm assembly across the disk surface at a very high speed to perform seek operations in which the head is positioned over a selected data track. The VCM also maintains the head over a selected track while reading or writing information. A servo system controller is the subsystem of the disk drive which is responsible for providing the head positioning necessary for reading and writing information in response to requests from a computer to which the disk drive is connected.
Along each track, the magnetic data are arranged consecutively about a centerline of the tracks. The data are generally organized into sectors or fields of predetermined length. A field of information is often preceded by a field of control information that may be used to verify the position of the head before a subsequent read or write operation. The data information fields may also include an error correction code ("ECC") which aids in correcting errors that may occur when information is read. In embedded servo disk drives, position verification and control information is contained in a servo field which is recorded on the tracks at the time of manufacture, utilizing a high precision servo writer or other techniques. The servo field information is used to perform continuous on-track positioning of the head with respect to the centerline of the track by reading and responding to the control information contained within the servo fields. The servo fields are interspersed with data fields in which the data information is recorded.
The servo control information typically includes a preamble which demarks the beginning of a servo field, a servo address mark ("SAM") which indicates that a valid servo field has been detected, a servo synch mark ("SSM") which is utilized to establish and maintain synchronization over reading and writing operations, an index mark which indicates a single reference point common to all the tracks or a band of tracks on the disk and a track number code, which is a Gray coded integer value of the track currently spanned by the read/write head.
The embedded servo field also typically includes off-track burst information which is written on the track when the disk drive is manufactured. The off-track bursts, which also comprise magnetic pulses, are physically positioned at precise intervals and locations with respect to the various track centerlines to provide the servo system controller with information relative to the fractional track-to-track displacement of the head with respect to a given track centerline. Normally, there are four off-track bursts, and the information obtained by reading the burst is sometimes referred to as quadrature signals, quadrature information or quadrature data. In the typical disk drive, the quadrature data are utilized by a data processor associated with a servo system controller to generate, calculate and provide control signals to the VCM to accurately position the head over the track centerline.
The servo control information in the servo field is commonly extracted from the head's signal, in conventional analog read channels, by an analog circuit which detects the presence of individual pulses. For example, U.S. Pat. No. 4,783,705 discloses an analog pulse detector circuit which detects peaks in the analog signal from the head (whether positive or negative in amplitude). These amplitude signals are then converted to digital signals and then passed to a servo controller. This technique is susceptible to noise in the channel and can erroneously detect two consecutive positive or negative pulses when, in magnetic recording, the pulses normally occur with alternating polarity.
Also in conventional analog read channels, the servo burst information in the servo field is typically extracted using an analog circuit that measures the servo burst amplitudes. These servo burst amplitudes are then processed by a motion control processor which generates control signals for positioning the read/write head. Typically, the amplitude of the off-track bursts is measured with analog peak detectors, which respond to the maximum of the head signal. Alternatively, the off-track bursts may be measured by analog area detectors (as in U.S. Pat. No. 4,783,705) which respond to the integrated amplitude of the head signal. In either case, the conventional burst amplitude measurement is generated by analog circuits and passed as an analog signal from a read channel integrated circuit ("IC") to an additional ADC in a separate servo controller.
Such conventional analog techniques for servo demodulation are inefficient for use in sampled amplitude read channels such as PRML read channels. Sampled amplitude read channels operate with discrete time circuits (and commonly digital circuits) which, being programmable, are highly configurable and adaptable. It is inefficient to incorporate the conventional analog servo demodulation circuits into a sampled amplitude read channel when programmable discrete time techniques can be implemented instead. Further, the discrete time circuitry already incorporated within a sampled amplitude read channel, such as an analog-to-digital converter and discrete time pulse detector, can also be used to implement demodulation of the servo data. Sharing the discrete time circuitry is a more cost effective implementation of servo demodulation since it requires less die area and less power. Finally, the prior art analog servo demodulation systems incorporated within a read channel cannot be programmably adapted to operate according to the various disk drives, data densities, and magnetic media found in the market. Nor can the prior art demodulation systems be programmably adapted to compensate for changes in the disk drive that occur over time.
Although for sampled amplitude read channels it is more economical to implement servo demodulation using discrete time circuitry, there are drawbacks which are overcome by the present invention. For example, the discrete time burst amplitude measurements are subject to inaccuracies due to variations in the sampling phase. Also, the burst amplitude measurement is subject to inaccuracies due to inconsistent timing of the burst detection signal. Further, the resolution of the channel ADC is inadequate for that required for servo demodulation.
Thus, a general object of the present invention is to demodulate servo control data in a magnetic storage system utilizing discrete time circuitry. Specifically, it is an object to provide discrete time servo demodulation in a sampled amplitude read channel IC. A further object is to share the discrete time circuitry already incorporated within a sampled amplitude read channel IC with the discrete time servo demodulation technique of the present invention, thereby minimizing the integrated circuitry and associated cost. Still another object is to transfer the servo field data to a servo controller through a wholly digital interface, thereby obviating the servo controller analog-to-digital converter. Yet another object is to implement servo demodulation utilizing programmable circuitry in order to adapt its operation to a particular disk drive system. Still a further object is to prevent the detection of two consecutive positive or negative pulse in the servo data. Another object is to increase the effective resolution of the channel ADC through various digital signal processing techniques. Still another object is to overcome inaccuracies in the burst amplitude discrete time measurement caused by variations in the sapling phase. A final object is to control the timing of the burst amplitude measurement so that the head signal is sampled over an integer number of servo burst cycles.